Analog-to-digital converters (ADCs) convert time-discrete analog input values to a digital form. A type of ADC, the successive approximation register (SAR) ADC, digitizes the analog input values using a successive approximation search algorithm. While the internal circuitry of the SAR ADC may run at a higher frequency (such as several megahertz (MHz), for example), the sample rate of the SAR ADC is generally a fraction of that frequency (such as several kilohertz (kHz), for example) due to the successive approximation search algorithm used. For example, normally each bit of the SAR ADC is fully realized prior to proceeding on to the next bit.
In general, many of today's microcontroller products can have several SAR ADCs on a single chip. In some high-end products, up to 20 or more ADC instances may be placed on the chip. To reduce the amount of ADC instances, a passive sample and hold (SH) ADC structure can be used with a time multiplexed operating scheme.
In some cases, an external voltage reference may be provided to the ADC. This external reference can be a direct supply to the ADC and also a supply to the sensors of the application, for instance. In such an application, the output of the sensors can be a ratiometric voltage based on the supply, which is converted by the ADC to a digital form for use by the microcontroller. The absolute value of the reference/supply voltage is generally cancelled in this scheme, making the external ADC reference desirable. In another implementation, the voltage reference may be provided by the ADC.
However, gain error can occur within passive SH ADCs, due to the use of two different capacitors: one for analog input sampling and one for the conversion process (internal digital-to-analog conversion). The gain error is associated with a mismatch of these two capacitors. In some cases, an on-chip reference voltage can provide at least a partial remedy for the gain error. Generally though, an on chip reference uses a reference buffer for the switched capacitor load of the ADC. This buffer uses chip area and current, and can produce noise. Further, the use of an on-chip reference disallows many of the benefits of an external reference, including those mentioned above.